RESEARCH ARTICLES Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.Science (Impact Factor: 33.61). 07/2012; 337(6090):54-8. DOI: 10.1126/science.1218740
Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.
Journal of Marine Systems 10/2015; DOI:10.1016/j.jmarsys.2015.10.011 · 2.51 Impact Factor
- "Phytoplankton concentration, a major component of marine biogeochemical cycles (Broecker et al., 1982; Sarmiento and Gruber, 2006), is driven by the availability of nutrients in the euphotic zone (Valiela, 1995). Known to be influenced by climatological processes such as wind speed, eddies and fronts, and pathways of currents (Taylor and Ferrari, 2011; Mahadevan et al., 2012), phytoplankton concentrations play an important role in the variability of oceanic carbon cycles (Thomalla et al., 2011). The principal photosynthetic pigment in phytoplankton, chlorophyll-a (hereafter, Chl-a), is the most widely used indicator for phytoplankton abundance. "
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- "Although increases in computing power mean that the resolution of models is always improving, Earth system models still poorly resolve features at scales of 100 km and smaller. This is unfortunate, as this regime is one where timescales of the physical circulation—which transport, mix, and disperse nutrients and plankton—are close to those of the ecological interactions within them, and the literature is increasingly well stocked with evidence for the significant ways in which eddies, fronts, filaments, and their ilk can influence biogeochemistry from local to global scales [e.g., McGillicuddy et al., 2007; Frajka-Williams et al., 2009; Mahadevan et al., 2012; Lévy et al., 2012a]. There are widely used techniques for representing the influence of sub–grid scale physical processes on the ocean circulation and tracers carried by it (hereafter " eddy transports " ) but only preliminary studies [e.g., Wallhead et al., 2013] dealing with what we will call here the " eddy reaction " terms. "
ABSTRACT: Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km -1 km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non-linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.Global Biogeochemical Cycles 08/2015; 29(9). DOI:10.1002/2015GB005129 · 3.97 Impact Factor
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- "Brody and Lozier (2014) parameterized vertical overturning time scales for the turbulent layer based on large-scale hydrographic properties and atmospheric forcings; again, they assumed that the turbulent motions were vertically uniform. Using higher-order turbulence closure models, Taylor and Ferrari (2011) and Mahadevan et al. (2012) are among the only studies to have employed a vertically varying diffusivity in their analyses of the SCD hypothesis. One of the points I make in this synthesis is that it is crucial to know not only the intensity of the turbulence, but also its vertical structure and temporal variability . "
ABSTRACT: Sverdrup (1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil International pour l'Exploration de la Mer, 18: 287–295) was quite careful in formulating his critical depth hypothesis, specifying a “thoroughly mixed top layer” with mixing “strong enough to distribute the plankton organisms evenly through the layer”. With a few notable exceptions, most subsequent tests of the critical depth hypothesis have ignored those assumptions, using estimates of a hydrographically defined mixed-layer depth as a proxy for the actual turbulence-driven movement of the phytoplankton. However, a closer examination of the sources of turbulence and stratification in turbulent layers shows that active turbulence is highly variable over time scales of hours, vertical scales of metres, and horizontal scales of kilometres. Furthermore, the mixed layer as defined by temperature or density gradients is a poor indicator of the depth or intensity of active turbulence. Without time series of coincident, in situ measurements of turbulence and phytoplankton rates, it is not possible to properly test Sverdrup's critical depth hypothesis.ICES Journal of Marine Science 08/2015; 72(6). DOI:10.1093/icesjms/fsu175 · 2.38 Impact Factor
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